In recent years, a high pressure acid leaching method using sulfuric acid has been gathering attention as a hydrometallurgical process for nickel oxide ore. This method is different from a dry smelting method that is a general smelting method for a nickel oxide ore of the related art and includes a continuous wet step without including dry steps such as reducing and drying steps. Thus, the method is advantageous in regard to energy and cost. In addition, the method is also advantageous in that it is possible to obtain a sulfide containing nickel (hereinafter, also referred to as “nickel sulfide”), whose nickel grade is improved to about 50% by mass (hereinafter, “% by mass” is simply referred to as “%”). The nickel sulfide is precipitated and generated through processes in which, after washing a leachate obtained by leaching the nickel oxide ore, by blowing a hydrogen sulfide gas thereto, a sulfuration reaction is caused to occur (a sulfuration step).
In a step for leaching metal from the nickel oxide ore by such a high temperature pressure acid leaching method (hereinafter, also simply referred to as “leaching step”), since impurity elements such as iron, magnesium, manganese, and aluminum are leached by sulfuric acid in addition to nickel and cobalt as recovery targets, an excessive amount of sulfuric acid is necessary for the treatment.
Further, in the sulfuration step for recovering nickel and cobalt, nickel and cobalt are selectively recovered as sulfides, but most of the impurity elements such as iron, magnesium, manganese, and aluminum leached by the leaching treatment in a leaching step do not form sulfides and remain in a barren solution obtained after sulfides are separated. In order to discharge this barren solution, it is necessary in a final neutralization step that metal ions remaining in the barren solution are precipitated and removed by a neutralization treatment.
Herein, in the final neutralization step, a method is generally performed in which the pH of the barren solution is increased to about 5 by adding a limestone slurry to the barren solution obtained through the sulfuration step so as to remove iron and aluminum and then the pH is increased to about 9 by adding a slaked lime slurry thereto so as to remove magnesium and manganese. Therefore, since the necessary amount (added amount) of the slaked lime slurry is determined depending on the amounts of magnesium ions and manganese ions remaining in the barren solution, a large amount of slaked lime slurry is needed in a case where the content of magnesium and the content of manganese in the nickel oxide ore are large.
Patent Document 1 discloses a technique of providing a simple and highly efficient smelting method as the entire process by simplification of a leaching step and a solid-liquid separation step, reducing the amount of neutralizer consumed in a neutralization step and the amount of a precipitate, an efficient method of repeatedly using water, and the like in a hydrometallurgical process for recovering nickel from a nickel oxide ore on the basis of high temperature pressure leaching. However, Patent Document 1 does not disclose the technical idea for reducing the amount of sulfuric acid used in the leaching treatment in the leaching step or reducing the amount of slaked lime used in the aforementioned final neutralization step.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2005-350766